Stanmeyer said the customer returned and shot the employee, killing him.

Authorities said a 28-year-old man was arrested at his home on Milwaukee's north side Friday night.

Police identified the victim as 21-year-old Reggie Evans from Milwaukee.

"I have a 21-year-old son. And I can't imagine what the family of this 21-year-old man is going through right now," Milwaukee Mayor Tom Barrett said.

Maria Trotter said she normally works side by side with Evans as a cook, but she had the day off and later learned of the attack.

"We don't have security, never had. We work third shift, and so we are the tougher crew," Trotter said. "Normally, we are able to hold our own, but somebody was so childish and sinister to come back with a gun and shoot him."

Trotter said Evans moved to Milwaukee from Detroit and was working two jobs to make ends meet.

Barrett went to the scene Friday afternoon with Police Chief Ed Flynn, who said the victim was helping waitresses who were having trouble with the rowdy group, telling them they had to leave.

"They had apparently been drinking and carousing somewhere. The activity was described as unruly and resulted in them throwing things around the restaurant and generally behaving very badly, and that's why the wait staff intervened," Flynn said.

But it sadly didn't end there.

"There was a struggle on the scene as the cook intervened with the waitresses to try to eject the unruly patrons. It was one of those patrons who returned with a firearm and murdered the cook," Flynn said.

Flynn said the people with the shooter identified him, and police scoured the city for him.

"It's a thriving commercial district, and I appreciate the response Capt. Rapp is going to provide in terms of reassuring people that the police are here. But frankly, the priority right now is not Oakland Avenue, the priority is finding the murderer," Milwaukee Alderman Nik Kovac said.

Police did not name the suspect but said he has a lengthy criminal record.

Barrett said the Milwaukee Police Department is devoting a lot of resources to this case.